A river near a Russian city inside the Arctic Circle has turned red, according to residents and local media outlets.

Photos posted on Russian social media websites show the Daldykan River, located near Norilsk, colored a deep red, ABC News reported. Russian officials have not been able to come up with a reason for the phenomenon.

Russia’s Environment Ministry said it will investigate a metals plant upstream.

The Nadezhdinsky factory, also known as the Nadezhda Metallurgical Plant, which possess nickel concentrate, has been suggested as the source of the coloration, the Siberian Times reported.

Norilsk Nickel owns the factory, and it has questioned claims its plant caused any pollution to the river. An investigation is underway, it said.

“However, environmental monitoring around the river and adjoined production facilities of the company is being held, including helicopter flights,” a spokesman for the firm told the Times.

But Denis Koshevoi, a PhD candidate at the Vernadsky Institute for Geochemistry and Analytical Chemistry, told The Guardian: “Periodically there are accidents when these pipes break and the solutions spill and get into the Daldykan—that’s why it changes colour.”

A social media user, Evgeny Belikov, claimed to have worked at the Nadezhdinsky plant, saying workers referred to a reservoir connected to it as the “red sea,” adding that it was named as such due to its color, which is produced by ore runoff, ABC News reported.

“In winter, the snow’s also red,” Belikov wrote. “On the one hand, it’s beautiful, but on the other, it’s chemical.”

Norilsk is considered the world’s northernmost city with more than 100,000 residents.

Pollution in the city is a major problem. The Blacksmith Institute named Norilsk as one of the top ten most polluted cities on earth. The Russian Federal State Statistics Service also named the city the most polluted in Russia